Method of form removal from precision casting shells



G SHELLS R. A. HORTON June 25, 1963 METHOD OF FORM REMOVAL FROM PRECISION CASTIN Filed Aug. 22, 1960 INVENTOR- A. HORTON 3% ROBERT 12% W W 3,094,751 Patented June 25, 1963 Fice 3,994,751 METHOD OF FORM REMOVAL FROM PRECISION CASTING SHELLS Robert A. Horton, Chesterland, Ohio, assignor to Precision Metalsmiths, Inc., Cleveland, Ohio, :1 corporation of Ohio Filed Aug. 22, 1960, Ser. No. 51,072 Claims. (Cl. 22-196) This invention relates to a ceramic shell process of investment casting, and relates more specifically to the use of plastic patterns and the process of removing such patterns from a shell coating formed thereon.

This invention is a continuation-in-part of application Serial No. 12,899 filed March 7, 1960, entitled, Method of Form Removal From Precision Casting Shells, now Patent No. 3,018,528, granted January 30, 1962.

A principal object of this invention is to provide a method of employing a shell mold pattern made of a soluble synthetic resin.

A related principal object of this invention is to provide a method of employing a synthetic resin pattern for forming a shell mold wherein the pattern can be removed without damage to the formed shell.

A more particular object of this invention is to provide a method of softening or dissolving a soluble synthetic resin pattern within a formed shell mold after it has been used to form the shell mold.

Still another object of this invention is to provide a pattern assembly for use in forming a shell mold which permits the pattern to be removed from the shell mold after the mold has been formed without damage to the mold.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawing, in which:

FIGURE 1 is a longitudinal sectional view of a pattern assembly which has been dipped in a refractory shell coating material; and,

FIGURE 2 is a longitudinal sectional view of a pattern assembly using a plastic form.

In the conventional investment casting or lost wax process, a disposable pattern, which is a replica of the part to be cast, including the necessary risers and gates, is

coated with a refractory slurry which hardens and forms the mold face. The bulk of the mold is formed by a refractory castable investment which is poured or vibrated around the coated pattern in a suitable container generally known as a flask, and whichsubsequently'sets to a strong hardened mass. p

The completed mold is then dried and heated to melt out the bulk of the pattern material. still higher to burn off or volatilize the last residue of the pattern material and thus prepare the mold to receive the molten metal. The coating of the pattern is usually carried out by dipping it into a suspension of fine, refractory It is then heated powder and suitable bonding liquid. The binder is one which is capable of hardening during drying at room conditions. After dipping, the excess slurry is drained off, and the pattern is sprinkled or sanded with coarser refractory particles to facilitate the binding between the coating and back-up investment.

A growing trend within the investment casting industry is the use of thin ceramic shell molds in place of the conventional bulky investment molds. The shell molds are usually prepared by repeating the dipping and sanding operation described above until a sufficient coating thickness is obtained to resist the stresses that occur in subsequent processes. The usual thickness of from of an inch to /2 of an inch although either thinner or heavier shells may be indicated for special situations.

Some of the advantages that are frequently obtained with shell molds over conventional investment molds include: (l) lighter molds, easier to handle; (2) increased permeability; (3) increased thermal shock resistance; (4) easier knock-out and clean up after casting; (5) no flasks required; and, (6) greater flexibility in processing.

A critical stage in the processing of such shells is the elimination of the pattern material. As the shell containing the pattern is heated, the usual pattern materials expand at a much higher rate than the shell so that the resultant expansion forces tend to crack the shell. This has been overcome in the case of wax patterns by a procedure known as flash dewaxing. In this procedure, the shell is placed directly into a furnace at an elevated temperature, i.e. 1600 to 1800 F. Under these conditions the heat transfer through the shell is so rapid that a surface skin of Wax melts before the bulk of the wax can heat up enough to crack the shell. As the bulk of the wax does heat up, the molten surface material either flows out of the mold or soaks into the shell. This provides a space to accommodate the bulk of the expanded wax so that it will not crack the shell.

Attempts to use synthetic resins, referred to by the trade as plastic, in place of wax patterns in conventional shell molding practice have not been successful due to the fact thatsynthetic resins do not respond to flash dewaxing as waxes do. Possibly the surface skin does not soften as it should, or if it does, it is not fluid enough to soak into the mold, with the result that the shell is still subject to a large pressure from the expanding material. In some cases strains in the plastic, which are set up during the injection cycle, may be released during heating causing a distortion of the plastic which may also crack the shell.

There are many instances where it is desirable to use plastic patterns. Plastic patterns can be turned out on automatic injection equipment at much higher production rates than can be obtained in conventional waxing injection operations. In addition, plastic patterns are not brittle, and do not crack at low temperatures or soften at high temperatures normally encountered.

In the previous application of which this case is a continuation-in-part, it was shown that a result similar to flash dewaxing could be obtained by being certain that the shell was sutliciently porous to allow the solvent to penetrate the shell and soften the surface of the plastic patthe trunk of this tree assembly. The combined pressures of large numbers of such individual nodules actually caused the entire side wall of the shell to break loose and hence destroy the entire tree. There were other similar limitations whenever certain sizes and configurations of shells were attempted.

Accordingly, with the concepts set forth in this invention, the entire plastic pattern is attacked from its root end wherein it is exposed to the interior of a hollow stem 'and softened or actually dissolved substantially away.

Thus the pattern is bored from within by a solvent action and any fluid pressures developed are able to be relieved through the root end. Furthermore, by this procedure the entire dipping and coating operation to build the shell can be carried out without regard to stopping at a condition wherein the shell is pervious to allow the penetration of a solvent. Thus, the entire operation is speeded considerably.

into such solvent.

lighter than the solvent.

This invention provides a means for using plastic patterns and overcomes the past encountered cracking of the mold in removing plastic patterns.

A suitable pattern assembly known in the trade as a tree is formed employing plastic patterns in the following manner:

Referring now to FIGURE 1, a hollow form 11 is provided which may be of metal or other suitable material. The form 11 is preferably tapered somewhat to facilitate removal later in the process and may be frusto-conically shaped as illustrated in FIGURE 1. A wax coating 12 is placed on the outside surface of the form 11 and allowed to harden.

One or more patterns 13 of the desired shape, including the necessary gates and risers are formed of a soluble synthetic resin. It has been found that polystyrene is a suitable material. Each of the patterns 13 has a gate 16 formed integrally with it. The patterns 13 are placed on the form 11 as follows: A heating iron or suitable substitute is used to melt the wax at the selected locations of insertion of the patterns; when the wax at each location has been melted, the pattern gate 16 is embedded into the wax. Although it may be pressed tightly up against the form it is usually not necessary to do so; when the selected nrunber of patterns have been placed on the form 11, the tree is ready for coating.

The coating process is Well known in the art and is used to form the ceramic shell for the casting of metal. A suitable dipping compound has been found to be a suspension of zirconium silicate and fused silica and a bonding liquid consisting mainly of colloidal silica sol with small amounts of an organic film former, a wetting agent, and a defoaming agent. The tree is dipped into the suspension, removed, and the excess allowed to drip off. The coating thus formed is allowed to air dry at room temperature. The coat is then sanded with a coarse fire-clay grog. The dipping and sanding operations are repeated until a coating of the desired. thickness has been built up. This constitutes the molding shell 15.

When the shell has built up the desired thickness, the form 11 is then removed. The removal is accomplished by heating the form just enough to loosen the wax coating surrounding it and the form will then slide out leaving the ceramic shell surrounding the plastic patterns and the wax shell. The use of a hollow form allows rapid interior heating without any substantial heating of the surrounding shell and plastic patterns .and promotes rapid removal. A solid form may be used and heat transmitted from its base. After the form has been removed, the remainder is heated to remove the wax and thus the ends of the patterns are then exposed to the internal cavity of the formed shell.

The patterns then can be readily removed by filling the cavity left by the form with a suitable solvent such as methyl ethyl ketone or by dropping the entire assembly The solvent will immediately commence attacking the pattern at the exposed face and continue Working inwardly until the pattern has been substantially completely softened or dissolved. Not all plastics used for patterns are fully soluble, but end exposure will at least soften the pattern to the extent that it will be incapable of expansion under heat to break the shell.

It has been found that the process of removing a soluble type of plastic can be materially speeded by using a type of solvent which will result in removed plastic material For example, trichloroethylene will cause the indicated plastic material to float to the surface of the solvent bath and thus create a convection current and thus expose the undissolved plastic to a constantly renewed fresh solvent solution.

After the plastic patterns have been exposed to the solvent, the shell is removed from the solvent and heated to between 1600 and 1800 F. which sets the shell and makes it suitable for the casting of metal. This heating at the elevated temperature performs the additional function of volatilizing and driving off any wax that may be left together with any remaining plastic.

Referring now to FIGURE 2, the form is made of a soluble synthetic resin similar to that of the patterns. The forming of the tree is accomplished as follows: The form is left uncoated and the end of the gate of each pattern is dipped into a suitable solvent such as methyl ethyl ketone which will soften it to some extent; it is then pushed against the form forming a bond which will hold the pattern in position; then the tree is dipped, dried, and sanded as described above; next the shell with the form and attached patterns is placed in a suitable solvent which may again-be methyl ethyl ketone or trichloroethylene and the form and the patterns dissolved out in what could be termed a chemical means for removing the form.

Following this step, the shell is heated between 1600 and 1800 R, which will set the shell and drive off any plastic that may be remaining.

In summary, it is believed that essentially this invention is a process by which a soluble synthetic resin can be used as a pattern to form a shell for investment casting.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts and performing the process may be resorted to without departing from the spirit andthe scope of the invention as hereinafter claimed.

What is claimed is:

1. The process of producing a molding shell for the casting of shapes therein comprising the steps of preparing a hollow shaped pattern support, embedding to said pattern support a pattern formed of a soluble synthetic resin, forming a refractory shell mold around said pattern, removing said pattern support from said mold to expose a portion of said pattern, thereafter applying a liquid resin solvent to at least said exposed pattern portion until said pattern is at least partially dissolved and is softened to a condition wherein the softened pattern has a low degree of expansion force under heat of intensity necessary to volatilize the pattern, said solvent being applied while maintaining said pattern at a temperature below that which would cause said pattern to expand and crack said mold, and then heating said mold above the heat volatilization temperature of said pattern to remove any remaining pattern material from said mold.

2,. The process as claimed in claim 1 wherein said pattern support is formed of a material soluble in said solvent and is removed from said mold by applying said solvent thereto.

3. The process as claimed in claim 1 wherein said solvent is denser than the synthetic resin of said pattern.

4. A shell molding process embodying the use of a pattern assembly including at least one synthetic resin pattern and a form having an outer surface of pattern supporting material, said process comprising embedding in a pattern supporting material the root end of a pattern formed of synthetic resin forming a refractory shell mold around said pattern and pattern supporting material, removing pattern supporting material from said mold to expose the root end of said pattern, thereafter applying a liquid resin solvent to the root end of said pattern to soften and at least partially dissolve said pattern to such an extent that the expansion forces exerted on the mold by the pattern material when it is heated will not crack said mold, said solvent being applied While maintaining said pattern at a temperature below that which would cause said pattern to expand and crack said mold, and finally heating the mold above the heat volatilization temperature of said pattern to remove any remaining pattern material from said mold.

5. The process as claimed in claim 4 wherein said solvent is denser than the synthetic resin of said pattern;

6. The process of preparing a molding shell for casting of shapes therein comprising the steps of preparing a form coated with a pattern supporting material, embedding in said pattern supporting material a portion of a pattern formed of synthetic resin, forming a refractory shell mold around the assembly as thus formed, removing said form from said mold, removing pattern supporting material from said mold to expose said pattern portion, applying a liquid resin solvent to said exposed pattern portion to soften and at least partially dissolve said pattern to such an extent that the expansion forces exerted on said mold by the pattern material when it is heated will not crack said mold, said solvent being applied while maintaining said pattern at a temperature below that which would cause said pattern to expand and crack said mold, and finally heating said mold above the heat volatilization temperature of said pattern to remove any remaining pattern material from said mold.

7. The process as claimed in claim 6 wherein said pattern supporting material is formed of wax.

8. The process of producing a molding shell for the casting of shapes therein comprising the steps of coating a hollow metal form with wax, embedding in said wax a portion of a pattern formed of synthetic resin, dipping the wax coated form and pattern in a suspension of refractory shell forming material to form a refractory shell mold suitable for casting metal, heating said form sufi"1- ciently to loosen the wax coating and removing said form from said mold, heating and removing said wax to expose said pattern portion, then applying a liquid resin solvent to the exposed pattern portion to soften and at least partially dissolve said pattern to such an extent that the expansion forces on said mold by the pattern material when it is heated will not crack said mold, said steps of heating said form and wax and said step of applying said solvent being carried out while maintaining said pattern at a temperature below that which would cause said pattern to expand and crack said mold, and finally heating said mold above the heat volatilization temperature of said pattern to remove any remaining pattern material from said mold.

9. The process of claim 8 wherein said solvent is applied by soaking said mold in a bath of liquid solvent.

10. The process as claimed in claim 9 wherein said solvent is denser than the synthetic resin of said pattern.

References Cited in the file of this patent UNITED STATES PATENTS 2,756,475 Hanink et a1 July 31, 1956 2,815,552 Turnbull et a1. Dec. 10, 1957 FOREIGN PATENTS 1,115,559 France Ian. 9, 1956 OTHER REFERENCES Doolittle: The Technology of Solvents and Plasticizers, copyright, 1954, by Union Carbide and Carbon Corp., TP 247.5, D6, C2, N.Y., John Wiley and Sons, Inc., p. 452 relied upon. 

1. THE PROCESS OF PRODUCING A MOLDING SHELL FOR THE CASTING OF SHAPES THEREIN COMPRISING THE STEPS OF PREPARING A HOLLOW SHAPED PATTERN SUPPORT, EMBEDDING TO SAID PATTERN SUPPORT A PATTERN FORMED OF A SOLUBLE SYNTHETIC RESIN, FORMING A REFRACTOR SHELL MOLD AROUND SAID PATTERN, REMOVING SAID PATTERN SUPPORT FROM SAID MOLD TO EXPOSE A PORTION OF SAID PATTERN, THEREAFTER APPLYING A LIQUID RESIN SOLVENT TO AT LEAST SAID EXPOSED PATTERN PORTION UNTIL SAID PATTERN IS AT LEAST PARTIALLY DISSOLVED AND IS SOFTENED TO A CONDITION WHEREIN THE SOFTENED PATTERN HAS A LOW DEGREE OF EXPANSION FORCE UNDER HEAT OF INTENSITY NECESSARY TO VOLATILIZE THE PATTERN, SAID SLVENT BEING APPLIED WHILE MAINTAINING SAID PATTERN AT A TEMPERATURE BELOW THAT WHICH WOULD CAUSE SAID PATTERN TO EXPAND AND CRACK SAID MOLD, AND THEN HEATIN SAID MOLD ABOVE THE HET VOLATILIZATION TEMPERATURE OF SAID PATTERN TO REMOVE ANY REMAINING PATTERN MATERIAL FROM SAID MOLD. 